1. Field of the Invention
The present invention relates to a process for preparing polyether ester carbonate polyols by catalytic addition of carbon dioxide, alkylene oxides and cyclic anhydrides onto H-functional starter substances in the presence of double metal cyanide (DMC) catalyst.
2. Description of Related Art
The preparation of polyether carbonate polyols by catalytic reaction of alkylene oxides (epoxides) and carbon dioxide in the presence of H-functional starter substances (“starters”) has been the subject of intensive study for more than 40 years (e.g. Inoue et al., Copolymerization of Carbon Dioxide and Alkylenoxide with Organometallic Compounds; Die Makromolekulare Chemie 130, 210-220, 1969). This reaction is shown in schematic form in scheme (I), where R is an organic radical such as alkyl, alkylaryl or aryl, each of which may also contain heteroatoms, for example O, S, Si, etc., and where e and f are each integers, and where the product shown here in scheme (I) for the polyether carbonate polyol should merely be understood in such a way that blocks having the structure shown may in principle be present in the polyether carbonate polyol obtained, but the sequence, number and length of the blocks and the OH functionality of the starter may vary, and it is not restricted to the polyether carbonate polyol shown in scheme (I). This reaction (see scheme (I)) is environmentally very advantageous, since this reaction constitutes the conversion of a greenhouse gas such as CO2 to a polymer. A further product formed here as an unwanted by-product is the cyclic carbonate shown in scheme (I) (for example, when R=CH3, propylene carbonate).

Activation in the context of this invention refers to a step in which a portion of alkylene oxide compound, optionally in the presence of CO2 and/or H-functional starter compound, is added to the DMC catalyst and then the addition of the alkylene oxide compound is interrupted, and a subsequent exothermic chemical reaction causes an evolution of heat to be observed which can lead to a temperature peak (“hotspot”), and the conversion of alkylene oxide and optionally CO2 can cause a pressure drop to be observed in the reactor. Optionally, the portion of the alkylene oxide compound can be added in a plurality of individual steps, in which case the occurrence of evolution of heat is generally awaited each time. The process step of activation comprises the period from commencement of the addition of the portion of alkylene oxide compound, which is optionally effected in the presence of CO2, to the DMC catalyst until the end of the evolution of heat. In the case of addition of a portion of the alkylene oxide compound in a plurality of individual steps, the process step of activation comprises all the periods during which the portions of the alkylene oxide compound, optionally in the presence of CO2, have been added stepwise until the end of the evolution of heat after the addition of the last portion of the alkylene oxide compound. In general, the activation step may be preceded by a step for drying the DMC catalyst and optionally the H-functional starter compound at elevated temperature and/or reduced pressure, optionally with passage of an inert gas through the reaction mixture.
EP-A 2 287 226 discloses the copolymerization of propylene oxide, maleic anhydride and carbon dioxide in the presence of double metal cyanide catalysts, where it is optionally also possible to add further monomers, for example anhydrides, to the polymerization.
Liu Y. et al., “Synthesis, characterization and hydrolysis of an aliphatic polycarbonate”, POLYMER, vol. 47, 2006, pages 8453-8461 discloses the terpolymerization of propylene oxide, carbon dioxide and maleic anhydride over polymer-supported bimetallic complexes. However, H-functional starter compounds and DMC catalysts are not used.
Database Caplus (Online) Chemical Abstracts Service, Columbus, Ohio, US; 15, Nov. 2011, Dong Xu et al., “Study on synthesis of a novel polyester polyol” discloses the preparation of hydroxyl-terminated polyether ester polyols by copolymerization of propylene oxide, maleic anhydride and carbon dioxide in the presence of double metal cyanide catalysts. Dong Xu et al., however, do not disclose activation of the DMC catalyst in the presence of a cyclic anhydride.
WO-A 2011/089120 discloses the copolymerization of propylene oxide and carbon dioxide in the presence of double metal cyanide catalysts, where the double metal cyanide catalyst can be activated in a multistage process. However, the addition of anhydrides in the activation steps is not disclosed.
U.S. Pat. No. 6,713,599 B1 discloses the copolymerization of propylene oxide and carbon dioxide in the presence of double metal cyanide catalysts.
EP-A 0 222 453 discloses a process for preparing polycarbonates from alkylene oxides and carbon dioxide using a catalyst system composed of DMC catalyst and a cocatalyst such as zinc sulfate. This polymerization is initiated by contacting a portion of the alkylene oxide with the catalyst system once. Only thereafter are the residual amount of alkylene oxide and the carbon dioxide metered in simultaneously. The amount of 60% by weight of alkylene oxide compound relative to the H-functional starter compound, as specified in EP-A 0 222 453 for the activation step in examples 1 to 7, is high and has the disadvantage that this constitutes a certain safety risk for industrial scale applications because of the high exothermicity of the homopolymerization of alkylene oxide compounds.
WO-A 2003/029325 discloses a process for preparing high molecular weight aliphatic polyether carbonate polyols (weight-average molecular weight greater than 30 000 g/mol), in which a catalyst from the group consisting of zinc carboxylate and multimetal cyanide compound is used, this catalyst being anhydrous and first being contacted with at least a portion of the carbon dioxide before the alkylene oxide is added. Final CO2 pressures of up to 150 bar place very high demands on the reactor and on safety. Even the excessively high pressure of 150 bar resulted in incorporation of only about 33% by weight of CO2 up to a maximum of 42% by weight of CO2 into the polymer. The examples detailed describe the use of a solvent (toluene) which has to be removed again by thermal means after the reaction, which leads to increased time and cost demands. Furthermore, the polymers, with a polydispersity of 2.7 or more, have a very broad molar mass distribution.